1. Field of the Invention
The present invention relates to subsurface well completion equipment and, more particularly, to a method and apparatus for sealing the junction between a main wellbore and at least one lateral wellbore drilled from the main wellbore.
2. Description of the Related Art
In recent years, those engaged in the exploration of oil and gas have determined that production from a central wellbore can be economically enhanced by drilling one or more lateral branch wellbores out from the central wellbore. To drill a lateral wellbore, an opening or window must first be cut through a casing in the central wellbore at a location where it is desired to start drilling the lateral wellbore. This can be done before or after the casing is lowered into the main wellbore. A mechanical diverter, commonly referred to as a whipstock, is then placed inside the main wellbore just below the casing window. If the casing window is to be cut after the casing is already cemented in place in the central wellbore, then a milling tool is guided by the whipstock to mill the casing window in the casing. A drilling tool is then lowered into the main wellbore and diverted into and through the casing window by the whipstock. The drilling tool then continues on to drill the lateral wellbore. When the lateral wellbore has been drilled, the drilling tool is removed, and a casing string is lowered into the main wellbore, through the casing window, and into the lateral wellbore. It is very important that a seal be established at the junction of the main wellbore casing and the lateral wellbore casing; the present invention is directed to this need.
One approach used heretofore to seal the junction has been to simply pump cement down the main wellbore casing, into the lateral wellbore casing, down to the end of the lateral wellbore casing, back up into the annulus between the lateral wellbore and the lateral wellbore casing, and to the annulus adjacent the junction between the main wellbore casing and the lateral wellbore casing. Others have attempted to seal the junction in a similar manner, but with substances other than cement. The apparatus and method of the present invention, as more fully described below, were developed to provide an improved approach to sealing the junction between a main and lateral wellbores. A further object of the apparatus and method of the present invention is to include a means of locating the junction between the main and lateral wellbores to enable reentry into the lateral wellbore.